This invention relates to the alloy art and has particular relationship to the thermomechanical processing of alloys which are used in nuclear reactors. The alloys with which this invention concerns itself is AISI 316 stainless steel but, for the use involved here, the composition of the alloy is maintained within more restricted limits than is usually specified for AISI 316 alloy (Handbook of Physics and Chemistry, 43rd Edition, Pg. 1542). The specified composition of this alloy in weight percent is presented in the following Table I:
TABLE I ______________________________________ Specified Composition ______________________________________ Chromium 17.00-18.00 Nickel 13.00-14.00 Carbon 0.040-0.060 Molybdenum 2.00-3.00 Manganese 1.50-2.00 Nitrogen 0.010 maximum Aluminum 0.050 maximum Arsenic 0.030 maximum Boron 0.0010 maximum Cobalt 0.050 maximum Columbium 0.050 maximum Copper 0.04 maximum Phosphorous 0.020 maximum Silicon 0.50-0.75 Sulphur 0.010 maximum Tantalum 0.020 maximum Vanadium 0.200 maximum Iron Balance ______________________________________
This invention is based on investigation and studies carried out with alloys identified as CN-13 and CK 25. The chemical composition of five specimens of heat 81592 of CN-13 in weight percent is presented in the following Table II:
TABLE II ______________________________________ #1 #2 #3 #4 #5 ______________________________________ C .055 .055 .054 .054 .053 Mn 1.67 1.63 1.66 1.69 1.67 Si .51 .52 .52 .50 .51 P .003 .004 .004 .002 .003 S .003 .004 .004 .003 .004 Cr 17.26 17.28 17.39 17.29 17.26 Ni 13.83 13.77 13.72 13.78 13.72 Mo 2.29 2.25 2.26 2.25 2.25 Cu &lt;.01 &lt;.01 &lt;.01 &lt;.01 &lt;.01 Co .01 .01 .03 .01 .02 V .01 .01 .01 .01 .01 Al &lt;.005 &lt;.005 &lt;.005 &lt;.005 &lt;.005 N .004 .004 .004 .004 .004 Cb &lt;.01 &lt;.01 &lt;.01 &lt;.01 &lt;.01 Ta &lt;.015 &lt;.015 &lt;.015 &lt;.015 &lt;.015 As &lt;.005 &lt;.005 &lt;.005 &lt;.005 &lt;.005 B .0007 .0005 .0008 .0005 &lt;.0005 ______________________________________
The range of the compositions of Table II is presented in the following Table III:
TABLE III ______________________________________ Chromium 17.26-17.39 Nickel 13.72-13.83 Carbon 0.053-0.055 Molybdenum 2.25-2.29 Manganese 1.63-1.69 Nitrogen 0.004 Aluminum &lt;0.005 Arsenic &lt;0.005 Boron 0.0007 Cobalt 0.01-0.03 Columbium &lt;0.01 Copper &lt;0.01 Phosphorous 0.002-0.004 Silicon 0.50-0.52 Sulfur 0.003-0.004 Tantalum &lt;0.015 Vanadium 0.01 Iron Balance ______________________________________
The chemical compositions of five specimens of heat 81615 of alloy CK-25 in weight percent is presented in the following Table IV:
TABLE IV ______________________________________ #1 #2 #3 #4 #5 ______________________________________ C .056 .057 .056 .056 .056 Mn 1.70 1.69 1.66 1.68 1.68 Si .51 .51 .50 .51 .50 P .002 .002 .002 .002 .002 S .003 .003 .003 .003 .003 Cr 17.56 17.51 17.46 17.48 17.54 Ni 13.84 13.82 13.67 13.75 13.82 Mo 2.34 2.32 2.296 2.28 2.32 Cu &lt;.01 &lt;.01 &lt;.01 &lt;.01 &lt;.01 Co .01 .01 .03 .01 .01 V &lt;.01 &lt;.01 &lt;.01 &lt;.01 &lt;.01 Al .011 .007 .007 .005 .005 N .004 .003 .004 .004 .004 B .0005 .0007 .0005 .0005 .0005 Cb &lt;.005 &lt;.005 &lt;.005 &lt;.005 &lt;.005 Ta &lt;.01 &lt;.01 &lt;.01 &lt;.01 &lt;.01 As &lt;.005 &lt;.005 &lt;.005 &lt;.005 &lt;.005 ______________________________________
The range of the compositions of Table IV is presented in the following Table V:
TABLE V ______________________________________ Chromium 17.46-17.56 Nickel 13.67-13.82 Carbon 0.056-0.057 Molybdenum 2.28-2.34 Manganese 1.66-1.70 Nitrogen 0.003-0.004 Aluminum 0.005-0.011 Arsenic &lt;0.005 Boron 0.0005-0.0007 Cobalt 0.01 Columbium &lt;0.005 Copper &lt;0.01 Phosphorous 0.002 Silicon 0.50-0.51 Sulfur 0.003 Tantalum &lt;0.01 Vanadium &lt;0.01 Iron Balance ______________________________________
Investigation of swelling was conducted with CN-13. The investigation and studies which led to this invention were conducted with heat 81615 of alloy CK-25.
The 316 alloy is used for cladding for the fuel pins, for ducts and also for other parts, such as control rod and absorber cladding, of nuclear reactors in which the fission is produced by neutrons in the epithermal energy range. The energy E of such neutrons are usually measured in units greater than one-tenth million electron volts (E&gt;0.1 MeV). Typically, epithermal neutrons produce fission in breeder reactors. In this application the parts and specimens which are investigated are referred to generally as articles.
Articles of 316 stainless steel, which are bombarded by neutrons in the epithermal range, swell. The effect of the neutron flux in producing the swelling is evaluated in terms of a function called the neutron fluence: EQU Neutron Fluence=.phi.T
where .phi. is the neutron flux and T the time of exposure of the article to the flux. The magnitude of the swelling influences the range of reactor environment and designed exposures in the continued use of the article. Where excessive swelling occurs, the reactor must be shut down and the affected article replaced.
The nuclear reactor articles composed of 316 stainless steel are made from ingots which after adequate processing are pressed into billets. The billets are converted by reduction and rolling and other treatment into the desired shapes and the resulting parts are reduced in size by cold working in a number of reducing steps. The billets and the components to which the billets are converted are also referred to herein as articles. Typically, the selected dimension (cross-sectional area) of each article is reduced to a small fraction of its initial magnitude. The article is subjected to eight to ten reductions by cold working to achieve the final dimension. Prior to each size reduction each article is cleaned, examined visually, annealed, pointed (the dimensions of the article which is being reduced is made smaller than the reducing die at one end) and lubricated. It has been known that swelling resulting from irradiation by neutron flux is reduced significantly from the annealed article if the last step reduces the cross-sectional area by 20%. The article thus produced is sometimes referred to as 20% cold-worked AISI 316 stainless steel. In accordance with the teachings of the prior art the minimum annealing temperature during reduction and particularly during the last reducing step is in excess of 1038.degree. C., typically about 1051.degree. C. or higher. Notwithstanding that swelling is reduced if the reduction is 20% during the last step, undesirable swelling has still been experienced in articles treated in accordance with the prior art.
It is an object of this invention to further minimize the swelling of articles made of AISI 316 stainless steel subjected to neutron fluence.